Semiconductor device and method for manufacturing the same

ABSTRACT

A more convenient and highly reliable semiconductor device which has a transistor including an oxide semiconductor with higher impact resistance used for a variety of applications is provided. A semiconductor device has a bottom-gate transistor including a gate electrode layer, a gate insulating layer, and an oxide semiconductor layer over a substrate, an insulating layer over the transistor, and a conductive layer over the insulating layer. The insulating layer covers the oxide semiconductor layer and is in contact with the gate insulating layer. In a channel width direction of the oxide semiconductor layer, end portions of the gate insulating layer and the insulating layer are aligned with each other over the gate electrode layer, and the conductive layer covers a channel formation region of the oxide semiconductor layer and the end portions of the gate insulating layer and the insulating layer and is in contact with the gate electrode layer.

TECHNICAL FIELD

The present invention relates to a semiconductor device and a method formanufacturing the semiconductor device.

In this specification, a semiconductor device generally means a devicewhich can function by utilizing semiconductor characteristics, and anelectro-optic device, a semiconductor circuit, and an electronic deviceare all semiconductor devices.

BACKGROUND ART

A technique for forming a thin film transistor (TFT) with the use of asemiconductor thin film formed over a substrate having an insulatingsurface has attracted attention. A thin film transistor is applied to awide range of electronic devices such as an integrated circuit (IC) oran image display device (display device).

As a material having semiconductor characteristics suitable for thinfilm transistors, a metal oxide has attracted attention, and thin filmtransistors in which a channel formation region is formed using such ametal oxide having semiconductor characteristics are known (see PatentDocuments 1 and 2).

In addition, electronic devices with the use of thin film transistorshave been used in a variety of places for a variety of applications andthus have been required to have various characteristics and shapes suchas light weight, thinness, and impact resistance. Accordingly,electronic devices having functions for serving their intended purposeshave been developed.

For example, as a semiconductor device which is provided for anamusement machine, a display whose display surface is curved so thatplayers can experience stereoscopic effect has been reported (e.g., seePatent Document 3).

REFERENCE [Patent Document]

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055-   [Patent Document 3] Japanese Published Patent Application No.    H7-114347

DISCLOSURE OF INVENTION

When semiconductor devices have a variety of shapes as described above,the semiconductor devices need to have high resistance to externalimpact.

In view of the above, it is an object of an embodiment of the presentinvention to provide a more highly impact-resistant semiconductor deviceincluding a transistor in which an oxide semiconductor is used.

Further, it is an object of an embodiment of the present invention toprovide a more convenient and highly reliable semiconductor device whichcan be used for a variety of applications.

According to an embodiment of the invention disclosed in thisspecification, a semiconductor device includes a bottom-gate transistorincluding a gate electrode layer, a gate insulating layer, and an oxidesemiconductor layer over a substrate, an insulating layer over thebottom-gate transistor, and a conductive layer over the insulatinglayer. The insulating layer is provided so as to cover the oxidesemiconductor layer and be in contact with the gate insulating layer. Ina direction of the channel width of the oxide semiconductor layer, anend portion of the gate insulating layer and an end portion of theinsulating layer are aligned with each other over the gate electrodelayer, and the conductive layer is provided so as to cover a channelformation region of the oxide semiconductor layer, the end portion ofthe gate insulating layer, and the end portion of the insulating layerand be in contact with the gate electrode layer.

According to an embodiment of the invention disclosed in thisspecification, a semiconductor device includes a driver circuit portionincluding a bottom-gate transistor for a driver circuit and a pixelportion including a transistor for a pixel over one substrate. Thebottom-gate transistor for a driver circuit includes a gate electrodelayer, a gate insulating layer, and an oxide semiconductor layer, aninsulating layer is provided over the oxide semiconductor layer, and aconductive layer is provided over the insulating layer. The insulatinglayer is provided so as to cover the oxide semiconductor layer and be incontact with the gate insulating layer. In a direction of the channelwidth of the oxide semiconductor layer, an end portion of the gateinsulating layer and an end portion of the insulating layer are alignedwith each other over the gate electrode layer, and the conductive layeris provided so as to cover a channel formation region of the oxidesemiconductor layer, the end portion of the gate insulating layer, andthe end portion of the insulating layer and be in contact with the gateelectrode layer.

In the above structure, a source electrode layer and a drain electrodelayer may be provided between the oxide semiconductor layer and theinsulating layer or between the gate insulating layer and the oxidesemiconductor layer.

In the above structure, in the channel width direction, the channelformation region of the oxide semiconductor layer is surrounded by thegate insulating layer and the insulating layer which are stacked belowand over the oxide semiconductor layer and also by the gate electrodelayer and the conductive layer; thus, the semiconductor device is highlyimpact-resistant and can have a variety of shapes by using a flexiblesubstrate as the substrate.

When a transistor including an oxide semiconductor layer is providedover a flexible substrate, a flexible semiconductor device can bemanufactured.

A transistor including an oxide semiconductor layer may be directlyformed over a flexible substrate. Alternatively, a transistor includingan oxide semiconductor layer may be formed over a manufacturingsubstrate, and then, the transistor may be separated and transferred toa flexible substrate. Note that in order to separate the transistor fromthe manufacturing substrate and transfer it to the flexible substrate, aseparation layer may be provided between the manufacturing substrate andthe transistor including the oxide semiconductor layer.

According to an embodiment of the invention disclosed in thisspecification, a method for manufacturing a semiconductor deviceincludes: forming a gate electrode layer over a flexible substrate,forming a gate insulating layer over the gate electrode layer, formingan oxide semiconductor layer over the gate insulating layer, forming aninsulating layer so as to cover the oxide semiconductor layer, formingan opening in the gate insulating layer and the insulating layer so asto expose the gate electrode layer, and forming a conductive layer so asto cover a top portion of a stack including the gate insulating layerand the insulating layer and cover an end portion of the stack includingthe gate insulating layer and the insulating layer and be in contactwith the gate electrode layer at the opening.

According to an embodiment of the invention disclosed in thisspecification, a method for manufacturing a semiconductor deviceincludes: forming a separation layer over a manufacturing substrate,forming a gate electrode layer over the separation layer, forming a gateinsulating layer over the gate electrode layer, forming an oxidesemiconductor layer over the gate insulating layer, forming aninsulating layer so as to cover the oxide semiconductor layer, formingan opening in the gate insulating layer and the insulating layer so asto expose the gate electrode layer, forming a conductive layer so as tocover a top portion of a stack including the gate insulating layer andthe insulating layer and cover an end portion of the stack including thegate insulating layer and the insulating layer and be in contact withthe gate electrode layer at the opening to form a transistor,transferring the transistor from the manufacturing substrate to asupporting substrate by using the separation layer, and transferring thetransistor that is transferred to the supporting substrate to a flexiblesubstrate.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for convenience and do not denote the order ofsteps and the stacking order of layers. In addition, in thisspecification, the ordinal numbers do not denote particular names whichspecify the invention.

According to an embodiment of the present invention, in a channel widthdirection, a channel formation region of an oxide semiconductor layer issurrounded by a gate insulating layer and an insulating layer which arestacked and also by a gate electrode layer and a conductive layer; thus,a semiconductor device can be impact-resistant.

According to an embodiment of the present invention, by being formed tobe flexible, a semiconductor device can be used for a variety ofapplications, and a more convenient and highly reliable semiconductordevice can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C illustrate an embodiment of a semiconductor device.

FIGS. 2A and 2B illustrate an embodiment of a semiconductor device.

FIGS. 3A1 to 3E2 illustrate an embodiment of a method for manufacturinga semiconductor device.

FIGS. 4A to 4D illustrate an embodiment of a method for manufacturing asemiconductor device.

FIGS. 5A to 5C illustrate an embodiment of a semiconductor device.

FIGS. 6A and 6B each illustrate an embodiment of a semiconductor device.

FIG. 7 illustrates an embodiment of a semiconductor device.

FIG. 8 illustrates an embodiment of a semiconductor device.

FIG. 9 illustrates an embodiment of a semiconductor device.

FIGS. 10A and 10B illustrate an electronic device.

FIG. 11 illustrates an electronic device.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details thereofcan be modified in various ways. Therefore, the present invention is notconstrued as being limited to the description of the embodiments below.

Embodiment 1

In this embodiment, an embodiment of a semiconductor device and a methodfor manufacturing the semiconductor device will be described withreference to FIGS. 1A to 1C and FIGS. 3A1 to 3E2. In this embodiment, atransistor will be described as an example of the semiconductor device.Note that an oxide semiconductor layer is preferable for use as asemiconductor layer in a semiconductor device disclosed in thisspecification.

As illustrated in FIGS. 1A to 1C, a channel formation region of an oxidesemiconductor layer 403 in a transistor 410 has a channel length (L)direction and a channel width (W) direction.

FIG. 1A is a plan view of the transistor 410, FIG. 1B is across-sectional view of the transistor 410 illustrated in FIG. 1A alongline A1-A2 in the channel length (L) direction, and FIG. 1C is across-sectional view along line B1-B2 in the channel width (W)direction.

As illustrated in FIGS. 1A to 1C, the transistor 410 includes, over asubstrate 400 having an insulating surface, a gate electrode layer 401,a gate insulating layer 402, the oxide semiconductor layer 403, a sourceelectrode layer 405 a, and a drain electrode layer 405 b. An insulatinglayer 407 and a conductive layer 411 are stacked in this order over thetransistor 410.

In the cross-sectional view in the channel width direction in FIG. 1C,the insulating layer 407 and the gate insulating layer 402 cover a topportion, a bottom portion, and end portions of the oxide semiconductorlayer 403 so as to surround the oxide semiconductor layer 403 and are incontact with each other at opposite end portions of the oxidesemiconductor layer 403. The gate electrode layer 401 is provided belowthe gate insulating layer 402, and the conductive layer 411 is providedover the insulating layer 407 so as to cover top portions of the oxidesemiconductor layer 403, the gate insulating layer 402, and theinsulating layer 407 and opposite end portions of the gate insulatinglayer 402 and the insulating layer 407. Moreover, the conductive layer411 is in contact with the gate electrode layer 401.

In the above manner, in the channel width direction, the oxidesemiconductor layer 403 is surrounded by the gate insulating layer 402and the insulating layer 407, and also by the gate electrode layer 401and the conductive layer 411.

With the structure as described above in which the periphery of theoxide semiconductor layer 403 is protected by a stack including the gateelectrode layer, the gate insulating layer, the insulating layer, andthe conductive layer, even when force (external force) is applied in thechannel width (W) direction as indicated by an arrow 445 in FIG. 1C, thethick stack structure is unlikely to be bent, so that force applied tothe oxide semiconductor layer 403 which is at the center of the stackcan be reduced. Accordingly, the oxide semiconductor layer 403 can beprevented from being broken owing to external impact.

In addition, an opening where the gate electrode layer 401 is exposedwidely is formed in the gate insulating layer 402 and the insulatinglayer 407, and the gate electrode layer 401 and the conductive layer 411are in contact with each other at the opening. By using a conductivefilm with high adhesion for each of the gate electrode layer 401 and theconductive layer 411, the gate electrode layer 401, the gate insulatinglayer 402, the oxide semiconductor layer 403, the insulating layer 407,and the conductive layer 411 can be prevented from peeling at theinterface thereof owing to the force indicated by the arrow 445.

In order to increase adhesion between the gate electrode layer 401 andthe conductive layer 411, a region where both the layers are in contactwith each other is preferably large. As illustrated in FIG. 1A, thelength of the region where the gate electrode layer 401 and theconductive layer 411 are in contact with each other in the direction ofthe channel length of the oxide semiconductor layer 403 is preferablylonger than the channel length of the oxide semiconductor layer 403.

The oxide semiconductor layer 403 is provided at the center, the gateinsulating layer 402 and the insulating layer 407 seal the oxidesemiconductor layer 403 by being in contact with each other at theopposite end portions, and further, the gate electrode layer 401 and theconductive layer 411 seal the above stack by being in contact with eachother at the opposite end portions, whereby a structure that issymmetric with respect to line C1-C2 can be obtained. With the abovestructure, the force indicated by the arrow 445 can be evenly dispersed,whereby local application of great force to the oxide semiconductorlayer 403 can be prevented.

Therefore, in the transistor 410, resistance to bending in the directionof the channel width of the oxide semiconductor layer 403 is improved,and thus the transistor 410 can be impact-resistant.

In a driver circuit, it is preferable that a transistor have a longchannel width so that a larger amount of current can flow. However, whenthe transistor has a long channel width, influence by external force inthe channel width direction is increased. Therefore, it is effective toapply a transistor which has resistance to bending in the channel widthdirection as described in this embodiment to the driver circuit in orderto achieve a highly impact-resistant and highly reliable semiconductordevice.

Since the transistor is impact-resistant, it can be applied to aflexible semiconductor device by using a flexible substrate for thesubstrate 400, and a more convenient and highly reliable semiconductordevice which can be used for a variety of applications can be provided.

Since the transistor disclosed in this specification is particularlyexcellent in resistance to bending in the direction of the channel widthof the oxide semiconductor layer, in manufacture of a semiconductordevice, it is preferable to form the transistor such that the channelwidth direction thereof matches a direction in which the semiconductordevice is likely to be bent (a direction in which the semiconductordevice is frequently bent).

FIGS. 3A1 to 3E2 illustrate an example of a method for manufacturing thetransistor 410. Note that FIGS. 3A1, 3B1, 3C1, 3D1, and 3E1 correspondto FIG. 1B, whereas FIGS. 3A2, 3B2, 3C2, 3D2, and 3E2 correspond to FIG.1C.

First, a conductive film is formed over the substrate 400 having aninsulating surface, and then, the gate electrode layer 401 is formed bya first photolithography step. Note that a resist mask may be formed byan inkjet method. Formation of the resist mask by an inkjet method needsno photomask; thus, manufacturing cost can be reduced.

For the substrate 400 having an insulating surface, a flexible substratecan be used. For example, a polyester resin such as polyethyleneterephthalate (PET) or polyethylene naphthalate (PEN), apolyacrylonitrile resin, a polyimide resin, a polymethyl methacrylateresin, a polycarbonate (PC) resin, a polyethersulfone (PES) resin, apolyamide resin, a cycloolefin resin, a polystyrene resin, a polyamideimide resin, or a polyvinylchloride resin is preferably used. Astructure body in which a fibrous body is impregnated with an organicresin (so-called prepreg) may also be used as a flexible substrate. Inaddition, over the substrate 400, a protective film having lowpermeability may be formed in advance, and examples thereof include afilm containing nitrogen and silicon such as a silicon nitride film or asilicon oxynitride film, a film containing nitrogen and aluminum such asan aluminum nitride film, and the like.

In the case where a fibrous body is contained in the material of thesubstrate 400, a high-strength fiber of an organic compound or aninorganic compound is used as the fibrous body. The high-strength fiberis specifically a fiber with a high tensile modulus of elasticity or afiber with a high Young's modulus. As a typical example of thehigh-strength fiber, a polyvinyl alcohol based fiber, a polyester basedfiber, a polyamide based fiber, a polyethylene based fiber, an aramidbased fiber, a polyparaphenylene benzobisoxazole fiber, a glass fiber,or a carbon fiber can be given. As the glass fiber, there is a glassfiber using E glass, S glass, D glass, Q glass, or the like. Thesefibers may be used in a state of a woven fabric or a nonwoven fabric,and a structure body in which such a fibrous body is impregnated with anorganic resin and the organic resin is cured may be used as thesubstrate 400. The structure body including the fibrous body and theorganic resin is preferably used as the substrate 400 becausereliability to bending or damaging due to local pressure can beincreased.

Alternatively, a glass substrate (such as a substrate of bariumborosilicate glass or aluminoborosilicate glass) which is thinned so asto have flexibility or a metal substrate which is processed into a filmmay be used. A material for forming the metal substrate is not limitedto a particular material, but aluminum, copper, nickel, an alloy ofmetals such as an aluminum alloy or stainless steel, or the like ispreferably used.

In order to manufacture a flexible semiconductor device, the transistor410 including the oxide semiconductor layer 403 may be directly formedover a flexible substrate. Alternatively, the transistor 410 includingthe oxide semiconductor layer 403 may be formed over a manufacturingsubstrate, and then, the transistor 410 may be separated and transferredto a flexible substrate. Note that in order to separate the transistorfrom the manufacturing substrate and transfer it to the flexiblesubstrate, a separation layer may be provided between the manufacturingsubstrate and the transistor including the oxide semiconductor layer.

An insulating film serving as a base film may be provided between thesubstrate 400 and the gate electrode layer 401. The base film has afunction of preventing diffusion of impurity elements from the substrate400, and can be formed with a single-layer structure or a stackstructure using one or more of a silicon nitride film, a silicon oxidefilm, a silicon nitride oxide film, and a silicon oxynitride film.

The gate electrode layer 401 can be formed to have a single-layerstructure or a stack structure using a metal material such asmolybdenum, titanium, tantalum, tungsten, aluminum, copper, neodymium,or scandium, or an alloy material which contains any of these materialsas a main component.

Next, the gate insulating layer 402 is formed over the gate electrodelayer 401. The gate insulating layer 402 can be formed with asingle-layer structure or a stack structure using one or more of asilicon oxide layer, a silicon nitride layer, a silicon oxynitridelayer, a silicon nitride oxide layer, an aluminum oxide layer, analuminum nitride layer, an aluminum oxynitride layer, an aluminumnitride oxide layer, and a hafnium oxide layer by a plasma CVD method, asputtering method, or the like.

The oxide semiconductor in this embodiment is an intrinsic (i-type) orsubstantially intrinsic (i-type) oxide semiconductor from which animpurity is removed and which is highly purified so as to contain animpurity that serves as a carrier donor and is a substance other thanthe main component of the oxide semiconductor as little as possible.

The highly purified oxide semiconductor layer contains extremely fewcarriers (close to zero), and the carrier concentration thereof is lowerthan 1×10¹⁴/cm³, preferably lower than 1×10¹²/cm³, more preferably lowerthan 1×10¹¹/cm³.

Since there are extremely few carriers in the oxide semiconductor layer,the off-state current of the transistor can be small. It is preferablethat the off-state current be as small as possible.

Such a highly purified oxide semiconductor is highly sensitive to aninterface state and interface electric charge; thus, an interfacebetween the oxide semiconductor layer and the gate insulating layer isimportant. For that reason, the gate insulating layer that is to be incontact with the highly purified oxide semiconductor needs to have highquality.

For example, high-density plasma CVD using microwaves (e.g., with afrequency of 2.45 GHz) is preferably adopted because an insulating layerformed can be dense and have high withstand voltage and high quality.When the highly purified oxide semiconductor and the high-quality gateinsulating layer are in close contact with each other, the interfacestate density can be reduced to obtain favorable interfacecharacteristics.

Needless to say, another film formation method such as a sputteringmethod or a plasma CVD method can be employed as long as the methodenables formation of a good-quality insulating layer as the gateinsulating layer. Moreover, an insulating layer whose film quality asthe gate insulating layer and whose characteristics of an interface withthe oxide semiconductor are improved by heat treatment performed afterthe formation may be used. In any case, any insulating layer that has areduced interface state density and can form a favorable interface withthe oxide semiconductor, as well as having good film quality as a gateinsulating layer, may be used.

Further, in order that hydrogen, hydroxyl group, and moisture might becontained in the gate insulating layer 402 and an oxide semiconductorfilm 440 as little as possible, it is preferable that, as pretreatmentbefore formation of the oxide semiconductor film 440, the substrate 400over which layers up to and including the gate electrode layer 401 areformed or the substrate 400 over which layers up to and including thegate insulating layer 402 are formed be preheated in a preheatingchamber of a sputtering apparatus so that impurities such as hydrogenand moisture adsorbed to the substrate 400 are eliminated and removed.As an evacuation unit provided in the preheating chamber, a cryopump ispreferable. Note that this preheating treatment can be omitted. Further,this preheating may also be performed on the substrate 400 over whichlayers up to and including the source electrode layer 405 a and thedrain electrode layer 405 b are formed, before the formation of theinsulating layer 407.

Next, over the gate insulating layer 402, the oxide semiconductor film440 with a thickness of greater than or equal to 2 nm and less than orequal to 200 nm, preferably greater than or equal to 5 nm and less thanor equal to 30 nm is formed (see FIGS. 3A1 and 3A2).

Note that before the oxide semiconductor film 440 is formed by asputtering method, powder substances (also referred to as particles ordusts) which are attached on a surface of the gate insulating layer 402are preferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering refers to amethod in which, without application of voltage to a target side, an RFpower source is used for application of voltage to a substrate side inan argon atmosphere to generate plasma in the vicinity of the substrateand modify a surface. Note that instead of an argon atmosphere, anitrogen atmosphere, a helium atmosphere, an oxygen atmosphere, or thelike may be used.

As an oxide semiconductor used for the oxide semiconductor film 440, anyof the following oxide semiconductors can be used: a four-componentmetal oxide such as an In—Sn—Ga—Zn—O-based oxide semiconductor, athree-component metal oxide such as an In—Ga—Zn—O-based oxidesemiconductor, an In—Sn—Zn—O-based oxide semiconductor, anIn—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxidesemiconductor, an Al—Ga—Zn—O-based oxide semiconductor, or aSn—Al—Zn—O-based oxide semiconductor, a two-component metal oxide suchas an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxidesemiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-basedoxide semiconductor, a Sn—Mg—O-based oxide semiconductor, or anIn—Mg—O-based oxide semiconductor, or an In—O-based oxide semiconductor,a Sn—O-based oxide semiconductor, or a Zn—O-based oxide semiconductor,and the like. SiO₂ may be contained in the above oxide semiconductor.Note that in this specification, for example, an In—Ga—Zn—O-based oxidesemiconductor film means an oxide film containing indium (In), gallium(Ga), and zinc (Zn), and there is no particular limitation on thestoichiometric proportion. The In—Ga—Zn—O-based oxide semiconductor filmmay contain an element other than In, Ga, and Zn.

In addition, for the oxide semiconductor film 440, a thin film of amaterial represented by the chemical formula, InMO₃(ZnO)_(m) (m>0), canbe used. Here, M represents one or more metal elements selected from Ga,Al, Mn, and Co. For example, M can be Ga, Ga and Al, Ga and Mn, Ga andCo, or the like.

In this embodiment, the oxide semiconductor film 440 is formed by asputtering method with the use of an In—Ga—Zn—O-based oxidesemiconductor target. In addition, the oxide semiconductor film 440 canbe formed by a sputtering method in a rare gas (typically argon)atmosphere, an oxygen atmosphere, or a mixed atmosphere containing arare gas and oxygen.

As a target for forming the oxide semiconductor film 440 by a sputteringmethod, for example, an oxide target containing In₂O₃, Ga₂O₃, and ZnO ata composition ratio (molar ratio) of 1:1:1 is used, and with the use ofthe target, an In—Ga—Zn—O film is formed. Without limitation to thematerial and the composition of the target described above, for example,an oxide target containing In₂O₃, Ga₂O₃, and ZnO at 1:1:2 [molar ratio]may be used.

The filling rate of the oxide target is higher than or equal to 90% andlower than or equal to 100%, preferably higher than or equal to 95% andlower than or equal to 99.9%. With the use of the metal oxide targetwith a high filling rate, a dense oxide semiconductor film can beformed.

It is preferable that a high-purity gas from which impurities such ashydrogen, water, hydroxyl group, or hydride are removed be used as asputtering gas for forming the oxide semiconductor film 440.

The substrate is held in a deposition chamber kept under reducedpressure, and the substrate temperature is set to higher than or equalto 100° C. and lower than or equal to 600° C., preferably higher than orequal to 200° C. and lower than or equal to 400° C. By forming the oxidesemiconductor film in a state where the substrate is heated, theconcentration of impurities in the formed oxide semiconductor film canbe reduced. In addition, damage by sputtering can be reduced. Then,residual moisture in the deposition chamber is removed, a sputtering gasfrom which hydrogen and moisture are removed is introduced, and theabove-described target is used, so that the oxide semiconductor film 440is formed over the substrate 400. In order to remove the residualmoisture from the deposition chamber, an entrapment vacuum pump, forexample, a cryopump, an ion pump, or a titanium sublimation pump ispreferably used. Further, an evacuation unit may be a turbo pump (turbomolecular pump) provided with a cold trap. From the deposition chamberwhich is evacuated with the cryopump, for example, a hydrogen atom, acompound containing a hydrogen atom such as water (H₂O) (morepreferably, also a compound containing a carbon atom), and the like areremoved, whereby the concentration of impurities in the oxidesemiconductor film that is formed in the deposition chamber can bereduced.

As one example of the deposition condition, the distance between thesubstrate and the target is 100 mm, the pressure is 0.6 Pa, the electricpower of a direct-current (DC) power source is 0.5 kW, and theatmosphere is an oxygen atmosphere (the proportion of the oxygen flowrate is 100%). Note that a pulse direct current power source ispreferable because powder substances (also referred to as particles ordusts) generated in the film formation can be reduced and the filmthickness can be uniform.

Next, the oxide semiconductor film 440 is processed into anisland-shaped oxide semiconductor layer by a second photolithographystep. A resist mask for forming the island-shaped oxide semiconductorlayer may be formed by an inkjet method. Formation of the resist mask byan inkjet method needs no photomask; thus, manufacturing cost can bereduced.

In the case where a contact hole is formed in the gate insulating layer402, a step of forming the contact hole can be performed at the sametime as processing of the oxide semiconductor film 440.

Note that the etching of the oxide semiconductor film 440 here may beperformed by dry etching, wet etching, or both dry etching and wetetching. For example, as an etchant used for wet etching of the oxidesemiconductor film 440, a solution obtained by mixing phosphoric acid,acetic acid, and nitric acid, ammonia hydrogen peroxide (hydrogenperoxide at 31 wt %: ammonia water at 28 wt %: water=5:2:2), or the likecan be used. In addition, ITO07N (produced by KANTO CHEMICAL CO., INC.)may be used.

Next, the oxide semiconductor layer is subjected to first heattreatment. The oxide semiconductor layer can be dehydrated ordehydrogenated by this first heat treatment. The temperature of thefirst heat treatment is set to higher than or equal to 400° C. and lowerthan or equal to 750° C., or higher than or equal to 400° C. and lowerthan the strain point of the substrate. In this embodiment, thesubstrate is put in an electric furnace which is a kind of heattreatment apparatus and heat treatment is performed on the oxidesemiconductor layer in a nitrogen atmosphere at 450° C. for one hour,and then water and hydrogen are prevented from entering the oxidesemiconductor layer with the oxide semiconductor layer not exposed tothe air. In this manner, an oxide semiconductor layer 441 is obtained(see FIGS. 3B1 and 3B2).

The heat treatment apparatus is not limited to an electric furnace, anda device for heating an object to be processed by heat conduction orheat radiation from a heating element such as a resistance heatingelement may be used. For example, a rapid thermal annealing (RTA)apparatus such as a gas rapid thermal annealing (GRTA) apparatus or alamp rapid thermal annealing (LRTA) apparatus can be used. An LRTAapparatus is an apparatus for heating an object to be processed byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas. For the high-temperature gas, an inert gas whichdoes not react with an object to be processed by heat treatment, such asnitrogen or a rare gas such as argon, is used.

For example, as the first heat treatment, GRTA by which the substrate istransferred and put in an inert gas heated to a high temperature of 650°C. to 700° C., heated for several minutes, and transferred and taken outof the inert gas heated to the high temperature may be performed.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in nitrogen or a rare gas suchas helium, neon, or argon. Alternatively, nitrogen or a rare gas such ashelium, neon, or argon which is introduced into the heat treatmentapparatus has a purity of higher than or equal to 6N (99.9999%),preferably higher than or equal to 7N (99.99999%) (that is, the impurityconcentration is lower than or equal to 1 ppm, preferably lower than orequal to 0.1 ppm).

Further, after the oxide semiconductor layer is heated in the first heattreatment, a high-purity oxygen gas, a high-purity N₂O gas, or anultra-dry air (the dew point is lower than or equal to −40° C.,preferably lower than or equal to −60° C.) may be introduced into thesame furnace. It is preferable that the oxygen gas or the N₂O gas doesnot contain water, hydrogen, and the like. Alternatively, the oxygen gasor the N₂O gas which is introduced into the heat treatment apparatus hasa purity of higher than or equal to 6N, preferably higher than or equalto 7N (that is, the impurity concentration in the oxygen gas or the N₂Ogas is lower than or equal to 1 ppm, preferably lower than or equal to0.1 ppm). By the action of the oxygen gas or the N₂O gas, oxygen whichis a main component of the oxide semiconductor and which has beenreduced at the same time as the step for removing impurities bydehydration or dehydrogenation is supplied, so that the oxidesemiconductor layer can be a highly purified and electrically i-type(intrinsic) oxide semiconductor layer.

The first heat treatment of the oxide semiconductor layer can beperformed on the oxide semiconductor film 440 before being processedinto the island-shaped oxide semiconductor layer. In that case, thesubstrate is taken out of the heat treatment apparatus after the firstheat treatment, and then a photolithography step is performed.

Note that the first heat treatment may be performed at any of thefollowing timings besides the above as long as it is after the oxidesemiconductor layer is formed; after the source electrode layer and thedrain electrode layer are formed over the oxide semiconductor layer; andafter the insulating layer is formed over the source electrode layer andthe drain electrode layer.

Further, the step of forming the contact hole in the gate insulatinglayer 402 may be performed either before or after the first heattreatment is performed on the oxide semiconductor film 440.

In addition, as the oxide semiconductor layer, an oxide semiconductorlayer having a crystal region (a single crystal region) with a largethickness, that is, a crystal region which is c-axis-alignedperpendicularly to a surface may be formed by performing depositiontwice and heat treatment twice, regardless of a material of a basecomponent such as an oxide, a nitride, or a metal. For example, a firstoxide semiconductor film with a thickness of greater than or equal to 3nm and less than or equal to 15 nm is deposited, and first heattreatment is performed in a nitrogen, oxygen, rare gas, or dry airatmosphere at higher than or equal to 450° C. and lower than or equal to850° C., preferably higher than or equal to 550° C. and lower than orequal to 750° C., so that the first oxide semiconductor film having acrystal region (including a plate-like crystal) in a region including asurface is formed. Then, a second oxide semiconductor film which has alarger thickness than the first oxide semiconductor film is formed, andsecond heat treatment is performed at higher than or equal to 450° C.and lower than or equal to 850° C., preferably higher than or equal to600° C. and lower than or equal to 700° C., so that crystal growthproceeds upward with the use of the first oxide semiconductor film as aseed of the crystal growth and the whole second oxide semiconductor filmis crystallized. In such a manner, the oxide semiconductor layer havinga crystal region with a large thickness may be formed.

Next, a conductive film to be a source electrode layer and a drainelectrode layer (including a wiring formed in the same layer as thesource electrode layer and the drain electrode layer) is formed over thegate insulating layer 402 and the oxide semiconductor layer 441. As theconductive film used for the source electrode layer and the drainelectrode layer, for example, a metal film containing an elementselected from Al, Cr, Cu, Ta, Ti, Mo, and W, a metal nitride filmcontaining any of the above elements as its component (a titaniumnitride film, a molybdenum nitride film, or a tungsten nitride film), orthe like can be used. Alternatively, a film of a high melting pointmetal such as Ti, Mo, or W or a metal nitride film of any of theseelements (a titanium nitride film, a molybdenum nitride film, or atungsten nitride film) may be stacked on one or both of a lower side oran upper side of a film of a metal such as Al or Cu. Alternatively, theconductive film used for the source electrode layer and the drainelectrode layer may be formed using a conductive metal oxide. As theconductive metal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zincoxide (ZnO), indium oxide-tin oxide alloy (In₂O₃—SnO₂, which isabbreviated to ITO in some cases), indium oxide-zinc oxide alloy(In₂O₃—ZnO), or any of these metal oxide materials in which siliconoxide is contained can be used.

A resist mask is formed over the conductive film by a thirdphotolithography step. Etching is selectively performed, so that thesource electrode layer 405 a and the drain electrode layer 405 b areformed. Then, the resist mask is removed.

Light exposure at the time of the formation of the resist mask by thethird photolithography step may be performed using ultraviolet light,KrF laser light, or ArF laser light. A channel length L of thetransistor that is completed later is determined by a distance betweenbottom ends of the source electrode layer and the drain electrode layer,which are adjacent to each other over the oxide semiconductor layer 441.In the case where light exposure is performed for a channel length L ofshorter than 25 nm, the light exposure at the time of the formation ofthe resist mask in the third photolithography step may be performedusing extreme ultraviolet light having an extremely short wavelength ofseveral nanometers to several tens of nanometers. In the light exposureusing extreme ultraviolet light, the resolution is high and the focusdepth is large. For these reasons, the channel length L of thetransistor that is completed later can also be longer than or equal to10 nm and shorter than or equal to 1000 nm, and the circuit can operateat higher speed.

In order to reduce the number of photomasks and the number of steps inphotolithography, an etching step may be performed using a resist maskformed with the use of a multi-tone mask which is a light-exposure maskthrough which light is transmitted to have a plurality of intensities. Aresist mask formed with the use of a multi-tone mask has a plurality ofthicknesses and further can be changed in shape by etching; therefore,the resist mask can be used in a plurality of etching steps forprocessing into different patterns. Therefore, a resist maskcorresponding to at least two kinds of different patterns can be formedby using one multi-tone mask. Thus, the number of light-exposure maskscan be reduced and the number of corresponding steps in photolithographycan be also reduced, whereby simplification of a process can berealized.

Note that it is preferable that etching conditions be optimized so asnot to etch and divide the oxide semiconductor layer 441 when theconductive film is etched. However, it is difficult to obtain etchingconditions in which only the conductive film is etched and the oxidesemiconductor layer 441 is not etched at all. In some cases, only partof the oxide semiconductor layer 441 is etched to be an oxidesemiconductor layer having a groove portion (a recessed portion) whenthe conductive film is etched.

In this embodiment, since a Ti film is used as the conductive film andan In—Ga—Zn—O-based oxide semiconductor is used as the oxidesemiconductor layer 441, ammonium hydrogen peroxide (a mixture ofammonia, water, and hydrogen peroxide) is used as an etchant.

Next, by plasma treatment using a gas such as N₂O, N₂, or Ar, water orthe like adsorbed to a surface of an exposed portion of the oxidesemiconductor layer may be removed. In the case where the plasmatreatment is performed, the insulating layer 407 which in contact withpart of the oxide semiconductor layer is formed without exposure to theair.

The insulating layer 407 can be formed to a thickness of at least 1 nmusing a method by which impurities such as water or hydrogen do notenter the insulating layer 407, such as a sputtering method asappropriate. When hydrogen is contained in the insulating layer 407,entry of the hydrogen into the oxide semiconductor layer or extractionof oxygen from the oxide semiconductor layer due to the hydrogen iscaused, thereby making the resistance of a backchannel of the oxidesemiconductor layer low (making the backchannel have n-typeconductivity), so that a parasitic channel might be formed. Therefore,it is important that a film formation method in which hydrogen is notused is employed in order to form the insulating layer 407 containing aslittle hydrogen as possible.

As the insulating layer 407, an inorganic insulating film such as asilicon oxide film, a silicon oxynitride film, an aluminum oxide film,or an aluminum oxynitride film can be typically used.

In this embodiment, a 200-nm-thick silicon oxide film is formed as theinsulating layer 407 by a sputtering method. The substrate temperaturein film formation may be higher than or equal to room temperature andlower than or equal to 300° C. and is 100° C. in this embodiment. Thesilicon oxide film can be formed by a sputtering method in a rare gas(typically argon) atmosphere, an oxygen atmosphere, or a mixedatmosphere containing a rare gas and oxygen. As a target, a siliconoxide target or a silicon target can be used. For example, the siliconoxide film can be formed using a silicon target by a sputtering methodin an atmosphere containing oxygen. As the insulating layer 407 which isformed in contact with the oxide semiconductor layer, an inorganicinsulating film which does not contain impurities such as moisture, ahydrogen ion, and OW and blocks entry of these from the outside is used.Typically, a silicon oxide film, a silicon oxynitride film, an aluminumoxide film, an aluminum oxynitride film, or the like is used.

In order to remove residual moisture from the deposition chamber of theinsulating layer 407 in a manner similar to that of the deposition ofthe oxide semiconductor film 440, an entrapment vacuum pump (such as acryopump) is preferably used. When the insulating layer 407 is depositedin the deposition chamber evacuated using a cryopump, the impurityconcentration of the insulating layer 407 can be reduced. In addition,as an evacuation unit for removing the residual moisture from thedeposition chamber of the insulating layer 407, a turbo pump (a turbomolecular pump) provided with a cold trap may be used.

It is preferable that a high-purity gas from which impurities such ashydrogen, water, hydroxyl group, or hydride are removed be used as asputtering gas when the insulating layer 407 is formed.

Next, second heat treatment is performed in an inert gas atmosphere oran oxygen gas atmosphere (preferably at higher than or equal to 200° C.and lower than or equal to 400° C., e.g., higher than or equal to 250°C. and lower than or equal to 350° C.). For example, the second heattreatment is performed in a nitrogen atmosphere at 250° C. for one hour.In the second heat treatment, part of the oxide semiconductor layer (achannel formation region) is heated while being in contact with theinsulating layer 407.

Through the above process, the first heat treatment is performed on theoxide semiconductor film, so that impurities such as hydrogen, moisture,hydroxyl group, or hydride (also referred to as a hydrogen compound) areintentionally removed from the oxide semiconductor layer. Additionally,oxygen which is one of main components of the oxide semiconductor and issimultaneously reduced in a step of removing an impurity can besupplied. Therefore, the oxide semiconductor layer is highly purifiedand is made to be an electrically i-type (intrinsic) oxide semiconductorlayer.

Through the above-described steps, the transistor 410 is formed (seeFIGS. 3C1 and 3C2).

When a silicon oxide layer having a lot of defects is used as theinsulating layer 407, with the heat treatment which is performed afterthe formation of the silicon oxide layer, impurities such as hydrogen,moisture, hydroxyl group, or hydride contained in the oxidesemiconductor layer can be diffused into the silicon oxide layer so thatimpurities in the oxide semiconductor layer can be further reduced.

A protective insulating layer may be formed over the insulating layer407. For example, a silicon nitride film is formed by an RF sputteringmethod. An RF sputtering method has superiority in mass production andthus is a preferable method for forming the protective insulating layer.As the protective insulating layer, an inorganic insulating film whichdoes not contain an impurity such as moisture and blocks entry of thesefrom the outside, such as a silicon nitride film or an aluminum nitridefilm, is used.

After the formation of the protective insulating layer, heat treatmentmay be further performed in the air at higher than or equal to 100° C.and lower than or equal to 200° C. for longer than or equal to 1 hourand shorter than or equal to 30 hours. This heat treatment may beperformed at a fixed heating temperature. Alternatively, the followingchange in the heating temperature may be conducted plural timesrepeatedly: the heating temperature is increased from a room temperatureto a temperature of higher than or equal to 100° C. and lower than orequal to 200° C. and then decreased to a room temperature.

Next, the gate insulating layer 402 and the insulating layer 407 areselectively removed, so that an opening 412 a and an opening 412 b wherethe gate electrode layer 401 is exposed are formed (see FIGS. 3D1 and3D2). As illustrated in FIG. 3D2, the opening 412 a and the opening 412b are formed in the gate insulating layer 402 and the insulating layer407 such that the gate insulating layer 402 and the insulating layer 407surround and seal the oxide semiconductor layer 403 which is provided atthe center in the direction of the channel width of the oxidesemiconductor layer 403. In this embodiment, the gate insulating layer402 and the insulating layer 407 are etched with the use of the samemask and thus their end portions are substantially aligned with eachother.

Then, a conductive film is formed over the insulating layer 407 andetched by a photolithography step, whereby the conductive layer 411 isformed (FIGS. 3E1 and 3E2). The conductive layer 411 is formed so as tocover at least the channel formation region of the oxide semiconductorlayer 403.

As illustrated in FIG. 3E2, the conductive layer 411 is formed so as tocover top portions and opposite end portions of the gate insulatinglayer 402 and the insulating layer 407 which surround the oxidesemiconductor layer 403 provided over the gate electrode layer 401 andbe in contact with the gate electrode layer 401 exposed at the openings.The conductive layer 411 is in contact with the gate electrode layer 401and thus has the same potential as the gate electrode layer 401.

By providing the conductive layer 411 having the same potential as thegate electrode layer 401, a parasitic channel due to leakage current canbe prevented from being formed in the backchannel of the transistor 410.

In addition, the conductive layer 411 functions to block an externalelectric field (particularly, to block static electricity), that is, toprevent an external electric field from acting on the inside (a circuitportion including the transistor). Such a blocking function of theconductive layer 411 can prevent variation in electric characteristicsof the transistor 410 due to the influence of an external electric fieldsuch as static electricity.

In the transistor 410 including the highly purified oxide semiconductorlayer 403 which is formed in accordance with this embodiment, the valueof a current in an off state (the value of the off-state current) can bereduced to a value of lower than 10 zA/μm for a channel width of 1 μm (avalue of lower than 100 zA/μm at 85° C.).

In addition, the transistor 410 which includes the oxide semiconductorlayer 403 can operate at high speed because it can achieve field-effectmobility that is relatively high. Therefore, by using the transistor ina pixel portion of a liquid crystal display device, a high-quality imagecan be provided. In addition, by using the transistor including thehighly purified oxide semiconductor layer, since a driver circuitportion and a pixel portion can be formed over one substrate, the numberof components of the semiconductor device can be reduced.

As described above, a channel formation region of an oxide semiconductorlayer is surrounded by a gate insulating layer and an insulating layerwhich are stacked and also by a gate electrode layer and a conductivelayer in a channel width direction; thus, a semiconductor device can beimpact-resistant.

In addition, by being formed to be flexible, a semiconductor device canbe used for a variety of applications, and a more convenient and highlyreliable semiconductor device can be provided.

Embodiment 2

In this embodiment, another embodiment of a semiconductor device will bedescribed with reference to FIGS. 2A and 2B. In this embodiment, atransistor will be described as an example of the semiconductor device.The same portions as those in Embodiment 1 and portions having functionssimilar to those in Embodiment 1 and the same steps as those inEmbodiment 1 and steps similar to those in Embodiment 1 may be handledas in Embodiment 1, and repeated description is omitted. In addition,detailed description of the same portions is not repeated.

FIGS. 2A and 2B illustrate transistors 420 a, 420 b, and 420 c which areconnected in parallel. By parallel connection of a plurality of thetransistors 420 a, 420 b, and 420 c, substantially the same effect as inthe case of a wide channel width can be obtained, and a larger amount ofcurrent can flow. When such a structure in which a plurality oftransistors are provided in parallel so as to share the channel width orthe like is used in combination, the circuit can be designed morefreely. The structure including the transistors 420 a, 420 b, and 420 cin which a larger amount of current can flow is preferably used fortransistors for a driver circuit in a driver circuit portion.

A channel formation region of each of oxide semiconductor layers 423 a,423 b, and 423 c of the transistors 420 a, 420 b, and 420 c,respectively, has a channel length (L) direction and a channel width (W)direction.

FIG. 2A is a plan view of the transistors 420 a, 420 b, and 420 c, andFIG. 2B is a cross-sectional view along line B3-B4 in the channel width(W) direction of the transistors 420 a, 420 b, and 420 c illustrated inFIG. 2A.

As illustrated in FIGS. 2A and 2B, the transistors 420 a, 420 b, and 420c include, over a substrate 400 having an insulating surface, a gateelectrode layer 421, a gate insulating layer 422 (gate insulating layers422 a, 422 b, and 422 c), the oxide semiconductor layers 423 a, 423 b,and 423 c, a source electrode layer 425 a, and a drain electrode layer425 b. An insulating layer 427 (insulating layers 427 a, 427 b, and 427c) and a conductive layer 431 are stacked in this order over thetransistors 420 a, 420 b, and 420 c.

The transistors 420 a, 420 b, and 420 c are connected in parallel andhave the gate electrode layer 421, the source electrode layer 425 a, andthe drain electrode layer 425 b in common.

In the cross-sectional view in the channel width direction in FIG. 2B,the gate insulating layers 422 a, 422 b, and 422 c and the insulatinglayers 427 a, 427 b, and 427 c cover top portions, bottom portions, andend portions of the oxide semiconductor layers 423 a, 423 b, and 423 cso as to surround the oxide semiconductor layers 423 a, 423 b, and 423 crespectively and are in contact with each other at opposite end portionsof the oxide semiconductor layers 423 a, 423 b, and 423 c respectively.The gate electrode layer 421 is provided below the gate insulatinglayers 422 a, 422 b, and 422 c, and the conductive layer 431 is providedover the insulating layers 427 a, 427 b, and 427 c so as to cover topportions of the oxide semiconductor layers 423 a, 423 b, and 423 c, thegate insulating layers 422 a, 422 b, and 422 c, and the insulatinglayers 427 a, 427 b, and 427 c and opposite end portions of the gateinsulating layers 422 a, 422 b, and 422 c and the insulating layers 427a, 427 b, and 427 c. Moreover, the conductive layer 431 is in contactwith the gate electrode layer 421.

In the above manner, the oxide semiconductor layers 423 a, 423 b, and423 c are surrounded by the gate insulating layers 422 a, 422 b, and 422c and the insulating layers 427 a, 427 b, 427 c respectively, and alsoby the gate electrode layer 421 and the conductive layer 431 in thechannel width direction.

With the structure as described above in which the peripheries of theoxide semiconductor layers 423 a, 423 b, and 423 c are protected by astack including the gate electrode layer, the gate insulating layers,the insulating layers, and the conductive layer, even when force isapplied in the channel width direction, the thick stack structure isunlikely to be bent, so that force applied to the oxide semiconductorlayers 423 a, 423 b, and 423 c which are at the center of the stack canbe reduced. Accordingly, the oxide semiconductor layers 423 a, 423 b,and 423 c can be prevented from being broken owing to external impact.

In addition, an opening where the gate electrode layer 421 is exposedwidely is formed in the gate insulating layer 422 (the gate insulatinglayers 422 a, 422 b, and 422 c) and the insulating layer 427 (theinsulating layers 427 a, 427 b, and 427 c), and the gate electrode layer421 and the conductive layer 431 are in contact with each other at theopening. By using a conductive film with high adhesion for each of thegate electrode layer 421 and the conductive layer 431, the gateelectrode layer 421, the gate insulating layers 422 a, 422 b, and 422 c,the oxide semiconductor layers 423 a, 423 b, and 423 c, the insulatinglayers 427 a, 427 b, and 427 c, and the conductive layer 431 can beprevented from peeling at the interface thereof owing to external force.

In order to increase adhesion between the gate electrode layer 421 andthe conductive layer 431, a region where both the layers are in contactwith each other is preferably large. As illustrated in FIG. 2A, thelength of the region where the gate electrode layer 421 and theconductive layer 431 are in contact with each other in the direction ofthe channel length of the oxide semiconductor layers 423 a, 423 b, and423 c is preferably longer than the channel length of the oxidesemiconductor layers 423 a, 423 b, and 423 c.

In the transistors 420 a, 420 b, and 420 c, the oxide semiconductorlayers 423 a, 423 b, and 423 c are provided at the center, the gateinsulating layers 422 a, 422 b, and 422 c and the insulating layers 427a, 427 b, and 427 c seal the oxide semiconductor layers 423 a, 423 b,and 423 c by being in contact with each other at the opposite endportions, and further the gate electrode layer 421 and the conductivelayer 431 seal the above stacks by being in contact with each other atthe opposite end portions, whereby structures that are symmetric can beobtained. With the above structure, the external force can be evenlydispersed, whereby local application of great force to the oxidesemiconductor layers 423 a, 423 b, and 423 c can be prevented.

Therefore, in the transistors 420 a, 420 b, and 420 c, resistance tobending in the direction of the channel width of the oxide semiconductorlayers 423 a, 423 b, and 423 c is improved, and thus the transistors 420a, 420 b, and 420 c can be impact-resistant.

Since the transistors are impact-resistant, they can be applied to aflexible semiconductor device by using a flexible substrate for thesubstrate 400, and a more convenient and highly reliable semiconductordevice which can be used for a variety of applications can be provided.

As described above, channel formation regions of oxide semiconductorlayers are surrounded by gate insulating layers and insulating layerswhich are stacked and also by a gate electrode layer and a conductivelayer in the channel width direction; thus, a semiconductor device canbe impact-resistant.

In addition, by being formed to be flexible, a semiconductor device canbe used for a variety of applications, and a more convenient and highlyreliable semiconductor device can be provided.

This embodiment can be implemented by being combined with any of otherembodiments as appropriate.

Embodiment 3

In this embodiment, another embodiment of a semiconductor device will bedescribed with reference to FIGS. 5A to 5C. In this embodiment, atransistor will be described as an example of the semiconductor device.An example in which a formation step and a structure of a sourceelectrode layer and a drain electrode layer are different from those ofthe transistor described in Embodiment 1 will be described. Therefore,the same portions as those in Embodiment 1 and portions having functionssimilar to those in Embodiment 1 and the same steps as those inEmbodiment 1 and steps similar to those in Embodiment 1 may be handledas in Embodiment 1, and repeated description is omitted. In addition,detailed description of the same portions is not repeated.

In Embodiments 1 and 2, the source electrode layer 405 a and the drainelectrode layer 405 b are provided between the oxide semiconductor layer403 and the insulating layer 407. In this embodiment, the sourceelectrode layer 405 a and the drain electrode layer 405 b are providedbetween the gate insulating layer 402 and the oxide semiconductor layer403.

FIG. 5A is a plan view of a transistor 430, FIG. 5B is a cross-sectionalview along line A5-A6 in the channel length (L) direction of thetransistor 430 illustrated in FIG. 5A, and FIG. 5C is a cross-sectionalview along line B5-B6 in the channel width (W) direction.

The transistor 430 illustrated in FIGS. 5A to 5C is a bottom-gatetransistor and includes, over a substrate 400, a gate electrode layer401, a gate insulating layer 402, a source electrode layer 405 a, adrain electrode layer 405 b, and an oxide semiconductor layer 403. Inaddition, an insulating layer 407 which covers the transistor 430 and isin contact with the oxide semiconductor layer 403 is provided.

In the transistor 430, the gate insulating layer 402 is provided on andin contact with the substrate 400 and the gate electrode layer 401, andthe source electrode layer 405 a and the drain electrode layer 405 b areprovided on and in contact with the gate insulating layer 402. Further,the oxide semiconductor layer 403 is provided over the gate insulatinglayer 402, the source electrode layer 405 a, and the drain electrodelayer 405 b.

In the cross-sectional view in the channel width direction in FIG. 5C,the gate insulating layer 402 and the insulating layer 407 cover a topportion, a bottom portion, and end portions of the oxide semiconductorlayer 403 so as to surround the oxide semiconductor layer 403 and are incontact with each other at opposite end portions of the oxidesemiconductor layer 403. The gate electrode layer 401 is provided belowthe gate insulating layer 402, and the conductive layer 411 is providedover the insulating layer 407, so as to cover top portions of the oxidesemiconductor layer 403, the gate insulating layer 402, and theinsulating layer 407 and opposite end portions of the gate insulatinglayer 402 and the insulating layer 407. Moreover, the conductive layer411 is in contact with the gate electrode layer 401.

In the above manner, the oxide semiconductor layer 403 is surrounded bythe gate insulating layer 402 and the insulating layer 407, and also bythe gate electrode layer 401 and the conductive layer 411 in the channelwidth direction.

With the structure as described above in which the periphery of theoxide semiconductor layer 403 is protected by a stack including the gateelectrode layer, the gate insulating layer, the insulating layer, andthe conductive layer, even when force (external force) is applied in thechannel width (W) direction, the thick stack structure is unlikely to bebent, so that force applied to the oxide semiconductor layer 403 whichis at the center of the stack can be reduced. Accordingly, the oxidesemiconductor layer 403 can be prevented from being broken owing toexternal impact.

In addition, an opening where the gate electrode layer 401 is exposedwidely is formed in the gate insulating layer 402 and the insulatinglayer 407, and the gate electrode layer 401 and the conductive layer 411are in contact with each other at the opening. By using a conductivefilm with high adhesion for each of the gate electrode layer 401 and theconductive layer 411, the gate electrode layer 401, the gate insulatinglayer 402, the oxide semiconductor layer 403, the insulating layer 407,and the conductive layer 411 can be prevented from peeling at theinterface thereof owing to the external force.

In order to increase adhesion between the gate electrode layer 401 andthe conductive layer 411, a region where both the layers are in contactwith each other is preferably large. As illustrated in FIG. 5A, thelength of the region where the gate electrode layer 401 and theconductive layer 411 are in contact with each other in the direction ofthe channel length of the oxide semiconductor layer 403 is preferablylonger than the channel length of the oxide semiconductor layer 403.

The oxide semiconductor layer 403 is provided at the center, the gateinsulating layer 402 and the insulating layer 407 seal the oxidesemiconductor layer 403 by being in contact with each other at theopposite end portions, and further the gate electrode layer 401 and theconductive layer 411 seal the above stack by being in contact with eachother at the opposite end portions, whereby a structure that issymmetric can be obtained. With the above structure, the external forcecan be evenly dispersed, whereby local application of great force to theoxide semiconductor layer 403 can be prevented.

Therefore, in the transistor 430, resistance to bending in the directionof the channel width of the oxide semiconductor layer 403 is improved,and thus the transistor 430 can be impact-resistant.

Since the transistor is impact-resistant, it can be applied to aflexible semiconductor device by using a flexible substrate for thesubstrate 400, and a more convenient and highly reliable semiconductordevice which can be used for a variety of applications can be provided.

This embodiment can be implemented by being combined with any of otherembodiments as appropriate.

Embodiment 4

In this embodiment, an example of a method for manufacturing asemiconductor device by separating a transistor from a manufacturingsubstrate and transferring the transistor to a flexible substrate willbe described. A semiconductor device according an embodiment of thepresent invention will be described with reference to FIGS. 4A to 4D.Note that this embodiment is the same as Embodiment 1 except part of theprocess; thus, the same portions are denoted by the same referencenumerals, and detailed description of the same portions is omitted.

An example of a method for manufacturing a semiconductor device will bedescribed in detail with reference to FIGS. 4A to 4D.

A separation layer 302 is formed over a first manufacturing substrate300 and a first insulating layer 301 is formed over the separation layer302. Preferably, the first insulating layer 301 is successively formedwithout exposing the separation layer 302 that is formed to the air.This successive formation prevents dusts or impurities from entering aninterface between the separation layer 302 and the first insulatinglayer 301.

As the first manufacturing substrate 300, a glass substrate, a quartzsubstrate, a sapphire substrate, a ceramic substrate, a metal substrate,or the like can be used. As the glass substrate, barium borosilicateglass, aluminoborosilicate glass, or the like can be used.Alternatively, a plastic substrate having heat resistance to theprocessing temperature of this embodiment may be used. In themanufacturing process of the semiconductor device, a manufacturingsubstrate can be selected as appropriate in accordance with the process.

Note that in this process, the separation layer 302 is formed over anentire surface of the first manufacturing substrate 300; however, afterforming the separation layer 302 over the entire surface of the firstmanufacturing substrate 300, the separation layer 302 may be selectivelyremoved so that the separation layer can be formed only over a desiredregion, if needed. Further, although the separation layer 302 is formedin contact with the first manufacturing substrate 300 in FIGS. 4A and4B, an insulating layer such as a silicon oxide layer, a siliconoxynitride layer, a silicon nitride layer, or a silicon nitride oxidelayer may be formed between the first manufacturing substrate 300 andthe separation layer 302, if needed.

The separation layer 302 has a single-layer structure or a stackstructure using an element selected from tungsten (W), molybdenum (Mo),titanium (Ti), tantalum (Ta), niobium (Nb), nickel (Ni), cobalt (Co),zirconium (Zr), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium(Os), iridium (Ir), and silicon (Si); an alloy material containing anyof the elements as a main component; or a compound material containingany of the elements as a main component. A crystal structure of a layercontaining silicon may be amorphous, microcrystalline, orpolycrystalline.

The separation layer 302 can be formed by a sputtering method, a plasmaCVD method, a coating method, a printing method, or the like. Note thata coating method includes a spin coating method, a droplet dischargemethod, and a dispensing method.

In the case where the separation layer 302 has a single-layer structure,it is preferable to form a tungsten layer, a molybdenum layer, or alayer containing a mixture of tungsten and molybdenum. Alternatively, alayer containing an oxide or an oxynitride of tungsten, a layercontaining an oxide or an oxynitride of molybdenum, or a layercontaining an oxide or an oxynitride of a mixture of tungsten andmolybdenum is formed. Note that the mixture of tungsten and molybdenumcorresponds to an alloy of tungsten and molybdenum, for example.

In the case where the separation layer 302 has a stack structure, atungsten layer, a molybdenum layer, or a layer containing a mixture oftungsten and molybdenum is preferably formed as a first layer. An oxideof tungsten, molybdenum, or a mixture of tungsten and molybdenum; anitride of tungsten, molybdenum, or a mixture of tungsten andmolybdenum; an oxynitride of tungsten, molybdenum, or a mixture oftungsten and molybdenum; or a nitride oxide of tungsten, molybdenum, ora mixture of tungsten and molybdenum is preferably formed as a secondlayer.

In the case where a stack structure of a layer containing tungsten and alayer containing an oxide of tungsten is formed as the separation layer302, the layer containing tungsten may be formed first, which isfollowed by the formation of an insulating layer formed using an oxideover the layer containing tungsten so that a layer containing an oxideof tungsten is formed at the interface between the layer containingtungsten and the insulating layer.

In addition, in the case where the transistor is formed over themanufacturing substrate with the separation layer therebetween, theseparation layer is heated by heat treatment for dehydration ordehydrogenation of an oxide semiconductor layer. Thus, when separationof the transistor from the manufacturing substrate and transfer thereofto a supporting substrate are performed in a later process, separationat the interface of the separation layer can be easily performed.

Alternatively, the layer containing an oxide of tungsten may be formedby performing thermal oxidation treatment, oxygen plasma treatment,treatment with a highly oxidizing solution such as ozone water, or thelike on the surface of the layer containing tungsten. The plasmatreatment and the heat treatment may be performed in an atmosphere ofoxygen, nitrogen, or dinitrogen monoxide alone, or a mixed gas of any ofthese gasses and another gas. The same can be applied to the case offorming a layer containing a nitride, an oxynitride, or a nitride oxideof tungsten. After a layer containing tungsten is formed, a siliconnitride layer, a silicon oxynitride layer, or a silicon nitride oxidelayer may be formed thereover.

A layer 304 to be separated is formed over the separation layer 302 (seeFIG. 4A). The layer 304 to be separated includes the first insulatinglayer 301 and a transistor 410.

First, the first insulating layer 301 is formed over the separationlayer 302. The first insulating layer 301 is preferably formed with asingle-layer or a multilayer using an insulating film containingnitrogen and silicon, such as silicon nitride, silicon oxynitride, orsilicon nitride oxide.

Further, the first insulating layer 301 can be formed by a sputteringmethod, a plasma CVD method, a coating method, a printing method, or thelike. For example, the first insulating layer 301 is formed by a plasmaCVD method at a temperature of 250° C. to 400° C., whereby a dense filmhaving very low water permeability can be obtained. Note that the firstinsulating layer 301 is formed to a thickness of greater than or equalto 10 nm and less than or equal to 1000 nm, preferably greater than orequal to 100 nm and less than or equal to 700 nm.

By forming the first insulating layer 301, separation can be easilyperformed at the interface between the layer 304 to be separated and theseparation layer 302 in a later separation process. Further, asemiconductor element or a wiring can be prevented from being cracked ordamaged in the later separation process. The first insulating layer 301serves as a protective layer of a semiconductor device.

The transistor 410 is formed over the first insulating layer 301, sothat the layer 304 to be separated is formed. The layer 304 to beseparated can be formed using the method described in Embodiment 1;accordingly, a detailed description thereof is omitted here.

In this embodiment, an example in which a protective insulating layer409 is stacked over an insulating layer 407 is described. In thisembodiment, as the protective insulating layer 409, a silicon nitridefilm is formed in such a manner that the substrate 400 over which layersup to and including the insulating layer 407 are formed is heated to atemperature of 100° C. to 400° C., a sputtering gas containinghigh-purity nitrogen from which hydrogen and moisture are removed isintroduced, and a target of a silicon semiconductor is used (see FIG.4A). In this case, the protective insulating layer 409 is preferablyformed while removing moisture remaining in a treatment chamber, in amanner similar to that of the insulating layer 407.

A planarization insulating film may be formed over the transistor 410 inorder to reduce surface roughness caused by the transistor. As theplanarization insulating film, an organic material such as polyimide,acrylic, or benzocyclobutene can be used. Other than such an organicmaterial, it is also possible to use a low-dielectric constant material(a low-k material) or the like. Note that the planarization insulatingfilm may be formed by stacking a plurality of insulating films formedfrom these materials.

Next, a second manufacturing substrate 306 is temporarily attached tothe layer 304 to be separated using an adhesive layer 305 which can beremoved. By attaching the second manufacturing substrate 306 to thelayer 304 to be separated, the layer 304 to be separated can be easilyseparated from the separation layer 302. In addition, it is possible tolower the stress added to the layer 304 to be separated in theseparation process, and the transistor can be protected. Further, sincethe adhesive layer 305 which can be removed is used, the secondmanufacturing substrate 306 can be easily removed when it is not neededany more.

As the adhesive layer 305 which can be removed, for example, awater-soluble resin can be used. Unevenness of the layer 304 to beseparated is reduced by applying the water soluble-resin, so that thelayer 304 to be separated can be easily attached to the secondmanufacturing substrate 306. In addition, as the adhesive layer 305which can be removed, a stack of a water-soluble resin and an adhesivecapable of being separated by light or heat may be used.

Next, the layer 304 to be separated is separated from the firstmanufacturing substrate 300 (see FIG. 4B). As a separation method,various methods can be employed.

For example, when a metal oxide film is formed as the separation layer302 on the side that is in contact with the first insulating layer 301,the layer 304 to be separated can be separated from the firstmanufacturing substrate 300 by weakening the metal oxide film bycrystallization. In addition, after the metal oxide film is weakened bycrystallization, part of the separation layer 302 may be removed byetching using a solution or a halogen fluoride gas such as NF₃, BrF₃, orClF₃ and separation may be performed at the weakened metal oxide film.

When a film containing nitrogen, oxygen, hydrogen, or the like (e.g., anamorphous silicon film containing hydrogen, an alloy film containinghydrogen, or an alloy film containing oxygen) is used as the separationlayer 302 and a substrate having a light-transmitting property is usedas the first manufacturing substrate 300, the following method can beused: the separation layer 302 is irradiated with laser light throughthe first manufacturing substrate 300, and nitrogen, oxygen, or hydrogencontained in the separation layer is evaporated, so that separation canoccur between the first manufacturing substrate 300 and the separationlayer 302.

Further, by removing the separation layer 302 by etching, the layer 304to be separated may be separated from the first manufacturing substrate300.

It is also possible to use a method for removing the first manufacturingsubstrate 300 by mechanical polishing, a method for removing the firstmanufacturing substrate 300 by etching using a halogen fluoride gas suchas NF₃, BrF₃ or ClF₃, or HF, or the like. In this case, the separationlayer 302 is not necessarily used.

Moreover, the layer 304 to be separated can be separated from the firstmanufacturing substrate 300 in the following manner: a groove to exposethe separation layer 302 is formed by laser light irradiation, byetching using a gas, a solution, or the like, or with a sharp knife orscalpel, so that separation occurs along the interface between theseparation layer 302 and the first insulating layer 301 serving as aprotective layer, with the groove used as a trigger.

For example, as a separation method, mechanical force (separationtreatment with a human hand or with a gripper, separation treatment byrotation of a roller, or the like) may be used. Alternatively, a liquidmay be dropped into the groove to allow the liquid to infiltrate intothe interface between the separation layer 302 and the first insulatinglayer 301, which may be followed by the separation of the layer 304 tobe separated from the separation layer 302. Further alternatively, amethod may be used in which a fluoride gas such as NF₃, BrF₃, or ClF₃ isintroduced into the groove and the separation layer 302 is removed byetching with the use of the fluoride gas so that the layer 304 to beseparated is separated from the first manufacturing substrate 300 havingan insulating surface. Further, the separation may be performed whilepouring a liquid such as water during the separation.

As another separation method, if the separation layer 302 is formedusing tungsten, separation can be performed while the separation layeris being etched by using a mixed solution of ammonia water and hydrogenperoxide.

Next, a substrate 400 is attached to the layer 304 to be separated usinga resin layer 307 (see FIG. 4C).

As the substrate 400, a flexible substrate as described in Embodiment 1can be used.

As the resin layer 307, various curable adhesives, e.g., a light curableadhesive such as a UV curable adhesive, a reactive curable adhesive, athermal curable adhesive, and an anaerobic adhesive can be used. As thematerial of the adhesive, an epoxy resin, an acrylic resin, a siliconeresin, a phenol resin, or the like can be used.

Note that in the case where a prepreg is used as the substrate 400, thelayer 304 to be separated and the substrate 400 are directly attached toeach other by pressure bonding without using an adhesive. At this time,as an organic resin for a structure body, a reactive curable resin, athermal curable resin, a UV curable resin, or the like which is bettercured by additional treatment is preferably used.

After providing the substrate 400, the second manufacturing substrate306 and the adhesive layer 305 which can be removed are removed, wherebythe transistor 410 is exposed (see FIG. 4D).

Through the above process, the transistor 410 can be formed over thesubstrate 400 by a transfer process.

Note that this embodiment shows an example of the method in whichcomponents up to and including the transistor are provided as the layerto be separated; however, the invention disclosed in this specificationis not limited thereto. The separation and transfer may be performedafter a display element (e.g., a light-emitting element) is formed.

According to this embodiment, a transistor manufactured using asubstrate having high heat resistance can be transferred to a flexiblesubstrate which is thin and lightweight. Therefore, a flexiblesemiconductor device can be formed without being restricted by the heatresistance of the substrate.

This embodiment can be implemented by being combined with any of otherembodiments as appropriate.

Embodiment 5

A semiconductor device with a display function (also referred to as adisplay device) can be manufactured using the transistor an example ofwhich is described in any of Embodiments 1 to 4. The transistor anexample of which is described in any of Embodiments 1 to 4 is moreeffectively used for a driver circuit portion. Further, by using thetransistor, part or whole of a driver circuit can be formed over thesame substrate as a pixel portion, whereby a system-on-panel can beobtained.

In FIGS. 6A and 6B, a sealant 4005 is provided so as to surround a pixelportion 4002 and a scan line driver circuit 4004 which are provided overa first substrate 4001. A second substrate 4006 is provided over thepixel portion 4002 and the scan line driver circuit 4004. Therefore, thepixel portion 4002 and the scan line driver circuit 4004 as well as adisplay element are sealed by the first substrate 4001, the sealant4005, and the second substrate 4006. In FIGS. 6A and 6B, a signal linedriver circuit 4003 which is formed over a substrate separately preparedusing a single crystal semiconductor film or a polycrystallinesemiconductor film is mounted in a region which is different from theregion surrounded by the sealant 4005 over the first substrate 4001. InFIGS. 6A and 6B, various signals and potential are supplied to thesignal line driver circuit 4003 which is separately formed, the scanline driver circuit 4004, and the pixel portion 4002 from an FPC 4018.

Although FIGS. 6A and 6B each illustrate the example in which the signalline driver circuit 4003 is formed separately and mounted on the firstsubstrate 4001, the present invention is not limited to this structure.The scan line driver circuit may be separately formed and then mounted,or only part of the signal line driver circuit or part of the scan linedriver circuit may be separately formed and then mounted.

Note that a connection method of a separately formed driver circuit isnot limited to a particular method, and a chip on glass (COG) method, awire bonding method, a tape automated bonding (TAB) method, or the likecan be used. FIG. 6A illustrates the example in which the signal linedriver circuit 4003 is mounted by a COG method. FIG. 6B illustrates theexample in which the signal line driver circuit 4003 is mounted by a TABmethod.

In addition, a display device includes in its category a panel in whicha display element is sealed, and a module in which an IC and the likeincluding a controller are mounted on a panel.

Note that a display device in this specification means an image displaydevice, a display device, or a light source (including a lightingdevice). Furthermore, the display device also includes the followingmodules in its category: a module to which a connector such as an FPC, aTAB tape, or a TCP is attached; a module having a TAB tape or a TCP atthe tip of which a printed wiring board is provided; and a module inwhich an integrated circuit (IC) is directly mounted on a displayelement by a COG method.

Further, the pixel portion and the scan line driver circuit which areprovided over the first substrate each include a plurality oftransistors, and the transistor an example of which is described in anyof Embodiments 1 to 4 can be used as the transistor for the scan linedriver circuit 4004.

As the display element provided in the display device, a liquid crystalelement (also referred to as a liquid crystal display element) or alight-emitting element (also referred to as a light-emitting displayelement) can be used. The light-emitting element includes, in itscategory, an element whose luminance is controlled by a current or avoltage, and specifically includes, in its category, an inorganicelectroluminescent (EL) element, an organic EL element, and the like.Furthermore, a display medium whose contrast is changed by an electriceffect, such as electronic ink, can be used.

Embodiments of a semiconductor device will be described with referenceto FIG. 7, FIG. 8, and FIG. 9. FIG. 7, FIG. 8, and FIG. 9 eachcorresponds to a cross-sectional view along M-N in FIG. 6A.

As illustrated in FIG. 7, FIG. 8, and FIG. 9, a semiconductor deviceincludes a connection terminal electrode 4015 and a terminal electrode4016, and the connection terminal electrode 4015 and the terminalelectrode 4016 are electrically connected to a terminal included in theFPC 4018 through an anisotropic conductive film 4019.

The connection terminal electrode 4015 is formed using the sameconductive film as a first electrode layer 4030, and the terminalelectrode 4016 is formed using the same conductive film as source anddrain electrode layers of transistors 4010 and 4011.

Each of the pixel portion 4002 and the scan line driver circuit 4004which are provided over the first substrate 4001 includes a plurality oftransistors. In FIG. 7, FIG. 8, and FIG. 9, the transistor 4010 includedin the pixel portion 4002 and the transistor 4011 included in the scanline driver circuit 4004 are illustrated as an example. In FIG. 7, aninsulating layer 4020 is provided over the transistors 4010 and 4011,and in FIG. 8 and FIG. 9, an insulating layer 4021 is provided over theinsulating layer 4020. Note that an insulating film 4023 is aninsulating film serving as a base film.

In this embodiment, for the transistor 4011 included in the scan linedriver circuit 4004, a transistor an example of which is described inEmbodiment 1 is used. In the transistor 4011, a channel formation regionof an oxide semiconductor layer is surrounded by a gate electrode layer,a gate insulating layer, an insulating layer, and a conductive layerprovided below and over the oxide semiconductor layer in a channel widthdirection. In a driver circuit, it is preferable that a transistor havea long channel width so that a larger amount of current can flow.Therefore, with the use of a transistor which has resistance to bendingin the channel width direction as described in any of Embodiments 1 to4, a highly impact-resistant and highly reliable semiconductor devicecan be obtained.

The transistor 4010 included in the pixel portion 4002 is electricallyconnected to a display element to form a display panel. A variety ofdisplay elements can be used as the display element as long as displaycan be performed.

FIG. 7 illustrates an example of a liquid crystal display device using aliquid crystal element for a display element. In FIG. 7, a liquidcrystal element 4013 which is a display element includes the firstelectrode layer 4030, a second electrode layer 4031, and a liquidcrystal layer 4008. Insulating films 4032 and 4033 serving as alignmentfilms are provided so that the liquid crystal layer 4008 is interposedtherebetween. The second electrode layer 4031 is provided on the secondsubstrate 4006 side, and the first electrode layer 4030 and the secondelectrode layer 4031 are stacked, with the liquid crystal layer 4008interposed therebetween.

Reference numeral 4035 indicates a columnar spacer formed by selectivelyetching an insulating film. The columnar spacer 4035 is provided inorder to control the thickness of the liquid crystal layer 4008 (a cellgap). Alternatively, a spherical spacer may be used.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Such a liquid crystal material exhibits acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on conditions.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while temperature of cholesteric liquidcrystal is increased. Since the blue phase appears only in a narrowtemperature range, a liquid crystal composition containing a chiralmaterial at higher than or equal to 5 wt. % is used for the liquidcrystal layer in order to improve the temperature range. The liquidcrystal composition containing a liquid crystal exhibiting a blue phaseand a chiral agent has a short response time of shorter than or equal to1 msec, has optical isotropy, which makes an alignment process unneeded,and has a small viewing angle dependence. In addition, since analignment film does not need to be provided and thus rubbing treatmentis unnecessary, electrostatic discharge damage caused by the rubbingtreatment can be prevented and defects and damage of a liquid crystaldisplay device can be reduced in the manufacturing process. Thus,productivity of the liquid crystal display device can be increased. Atransistor that includes an oxide semiconductor layer particularly has apossibility that electric characteristics of the transistor mayfluctuate significantly by the influence of static electricity anddeviate from the designed range. Therefore, it is more effective to usea blue phase liquid crystal material for a liquid crystal display deviceincluding a transistor including an oxide semiconductor layer.

The specific resistivity of the liquid crystal material is higher thanor equal to 1×10⁹ Ω·cm, preferably higher than or equal to 1×10¹¹ Ω·cm,more preferably higher than or equal to 1×10¹² Ω·cm. Note that thespecific resistivity in this specification is measured at 20° C.

The size of a storage capacitor formed in the liquid crystal displaydevice is set considering the leakage current of the transistor providedin the pixel portion or the like so that electric charge can be held fora predetermined period. The size of the storage capacitor may be setconsidering the off-state current of the transistor or the like. Byusing a transistor including a high-purity oxide semiconductor layer, itis enough to provide a storage capacitor having a capacitance that isless than or equal to ⅓, preferably less than or equal to ⅕ of a liquidcrystal capacitance of each pixel.

For the liquid crystal display device, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, anaxially symmetric aligned micro-cell (ASM) mode, an optical compensatedbirefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, anantiferroelectric liquid crystal (AFLC) mode, or the like can be used.

In addition, the liquid crystal display device may be a normally blackliquid crystal display device such as a transmissive liquid crystaldisplay device utilizing a vertical alignment (VA) mode. Some examplesare given as the vertical alignment mode; for example, a multi-domainvertical alignment (MVA) mode, a patterned vertical alignment (PVA)mode, an ASV mode, or the like can be employed. Furthermore, thisembodiment can be applied to a VA liquid crystal display device. The VAliquid crystal display device has a kind of form in which alignment ofliquid crystal molecules of a liquid crystal display panel is controlledas follows. In the VA liquid crystal display device, liquid crystalmolecules are aligned in a vertical direction with respect to a panelsurface when no voltage is applied. Moreover, it is possible to use amethod called domain multiplication or multi-domain design, in which apixel is divided into some regions (subpixels) and molecules are alignedin different directions in different regions.

Further, in the display device, a black matrix (a light-blocking layer),an optical member (an optical substrate) such as a polarizing member, aretardation member, or an anti-reflection member, and the like areprovided as appropriate. For example, circular polarization obtained byusing a polarizing substrate and a retardation substrate may be used. Inaddition, a backlight, a side light, or the like may be used as a lightsource.

As a display method for the pixel portion, a progressive method, aninterlace method, or the like can be employed. Further, color elementscontrolled in a pixel at the time of color display are not limited to acombination of three colors: R, G, and B (R, G, and B correspond to red,green, and blue, respectively). For example, a combination of R, G, B,and W (W corresponds to white); a combination of R, G, B, and one ormore of yellow, cyan, magenta, and the like; or the like can be used.Further, the sizes of display regions may be different depending on dotsof color elements. The present invention is not limited to a displaydevice for color display but can also be applied to a display device formonochrome display.

In addition, as the display element included in the display device, alight-emitting element utilizing electroluminescence can be used.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

As for the organic EL element, by application of voltage to alight-emitting element, electrons and holes are separately injected froma pair of electrodes into a layer containing a light-emitting organiccompound, and current flows. The carriers (electrons and holes) arerecombined, and thus, the light-emitting organic compound is excited.The light-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer in which particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure in which a light-emitting layer is sandwiched betweendielectric layers, which are further sandwiched between electrodes, andits light emission mechanism is localized type light emission thatutilizes inner-shell electron transition of metal ions. Note that anexample of an organic EL element is described here as a light-emittingelement.

In order to extract light emitted from the light-emitting element, atleast one of a pair of electrodes may be transparent. A transistor and alight-emitting element are formed over a substrate. The light-emittingelement can have a top emission structure in which light emission isextracted through the surface opposite to the substrate; a bottomemission structure in which light emission is extracted through thesurface on the substrate side; or a dual emission structure in whichlight emission is extracted through the surface opposite to thesubstrate and the surface on the substrate side, and a light-emittingelement having any of these emission structures can be used.

FIG. 8 illustrates an example of a light-emitting device in which alight-emitting element is used as a display element. A light-emittingelement 4513 which is a display element is electrically connected to thetransistor 4010 provided in the pixel portion 4002. Note that thestructure of the light-emitting element 4513 is, but not limited to, astack structure which includes the first electrode layer 4030, anelectroluminescent layer 4511, and a second electrode layer 4031. Thestructure of the light-emitting element 4513 can be changed asappropriate depending on a direction in which light is extracted fromthe light-emitting element 4513, or the like.

A partition wall 4510 can be formed using an organic insulating materialor an inorganic insulating material. It is particularly preferable thatthe partition wall 4510 be formed using a photosensitive resin materialto have an opening over the first electrode layer 4030 so that asidewall of the opening is formed as a tilted surface with continuouscurvature.

The electroluminescent layer 4511 may be formed using a single layer ora plurality of layers stacked.

A protective film may be formed over the second electrode layer 4031 andthe partition wall 4510 in order to prevent entry of oxygen, hydrogen,moisture, carbon dioxide, or the like into the light-emitting element4513. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC film, or the like can be formed. In addition, in aspace which is formed with the first substrate 4001, the secondsubstrate 4006, and the sealant 4005, a filler 4514 is provided forsealing. It is preferable that the light-emitting device be packaged(sealed) with a protective film (such as a laminate film or anultraviolet curable resin film) or a cover material with highair-tightness and little degasification so that the light-emittingdevice is not exposed to the outside air, in this manner.

As the filler 4514, an ultraviolet curable resin or a thermosettingresin as well as an inert gas such as nitrogen or argon can be used. Forexample, polyvinyl chloride (PVC), acrylic, polyimide, an epoxy resin, asilicone resin, polyvinyl butyral (PVB), or ethylene vinyl acetate (EVA)can be used. For example, nitrogen is used for the filler.

In addition, if needed, an optical film such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter, may be provided as appropriate on a light-emittingsurface of the light-emitting element. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducereflection can be performed.

Further, an electronic paper in which electronic ink is driven can beprovided as the display device. The electronic paper is also referred toas an electrophoretic display device (an electrophoretic display) and isadvantageous in that it has the same level of readability as plainpaper, it has lower power consumption than other display devices, and itcan be made thin and lightweight.

An electrophoretic display device can have various modes. Anelectrophoretic display device contains a plurality of microcapsulesdispersed in a solvent or a solute, each microcapsule containing firstparticles which are positively charged and second particles which arenegatively charged. By applying an electric field to the microcapsules,the first and second particles in the microcapsules move in oppositedirections to each other and only the color of the particles gatheringon one side is displayed. Note that the first particles and the secondparticles each contain a pigment and do not move without an electricfield. Moreover, the first particles and the second particles havedifferent colors (which may be colorless) from each other.

Thus, an electrophoretic display device is a display that utilizes aso-called dielectrophoretic effect by which a substance having a highdielectric constant moves to a high-electric field region.

A solution in which the above microcapsules are dispersed in a solventis referred to as electronic ink. This electronic ink can be printed ona surface of glass, plastic, cloth, paper, or the like. Furthermore, byusing a color filter or particles that have a pigment, color display canbe achieved.

Note that the first particles and the second particles in themicrocapsules may each be formed of a single material selected from aconductive material, an insulating material, a semiconductor material, amagnetic material, a liquid crystal material, a ferroelectric material,an electroluminescent material, an electrochromic material, and amagnetophoretic material, or formed of a composite material thereof.

Further, as the electronic paper, a display device in which a twistingball display system is employed can be used. The twisting ball displaysystem refers to a method in which spherical particles each colored inblack and white are arranged between a first electrode layer and asecond electrode layer which are electrode layers used for a displayelement, and a potential difference is generated between the firstelectrode layer and the second electrode layer to control orientation ofthe spherical particles, so that display is performed.

FIG. 9 illustrates an active matrix electronic paper as an embodiment ofa semiconductor device. The electronic paper in FIG. 9 is an example ofa display device using a twisting ball display system.

Between the first electrode layer 4030 connected to the transistor 4010and the second electrode layer 4031 provided on the second substrate4006, spherical particles 4613 each of which includes a black region4615 a, a white region 4615 b, and a cavity 4612 which is filled withliquid around the black region 4615 a and the white region 4615 b, areprovided. A space around the spherical particles 4613 is filled with afiller 4614 such as a resin. The second electrode layer 4031 correspondsto a common electrode (counter electrode). The second electrode layer4031 is electrically connected to a common potential line.

In FIG. 7, FIG. 8, and FIG. 9, as the first substrate 4001 and thesecond substrate 4006, flexible substrates, for example, plasticsubstrates having a light-transmitting property or the like can be used.As plastic, a fiberglass-reinforced plastics (FRP) plate, a polyvinylfluoride (PVF) film, a polyester film, or an acrylic resin film can beused. In addition, a sheet with a structure in which an aluminum foil issandwiched between PVF films or polyester films can be used.

The insulating layer 4020 functions as a protective film of thetransistors.

Note that the protective film is provided to prevent entry ofcontaminant impurities such as an organic substance, metal, or watervapor contained in the air and is preferably a dense film. Theprotective film may be formed with a single layer or a stack using oneor more of a silicon oxide film, a silicon nitride film, a siliconoxynitride film, a silicon nitride oxide film, an aluminum oxide film,an aluminum nitride film, an aluminum oxynitride film, and an aluminumnitride oxide film by a sputtering method.

The insulating layer 4021 functioning as a planarization insulating filmcan be formed using an organic material having heat resistance, such asacrylic, polyimide, benzocyclobutene, polyamide, or epoxy. Other thansuch organic materials, it is also possible to use a low-dielectricconstant material (a low-k material), a siloxane-based resin,phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or thelike. The insulating layer may be formed by stacking a plurality ofinsulating films formed of these materials.

There is no particular limitation on the method for forming theinsulating layers 4020 and 4021, and the insulating layers 4020 and 4021can be formed, depending on the material, by a sputtering method, an SOGmethod, spin coating, dipping, spray coating, or a droplet dischargemethod (such as an inkjet method, screen printing, or offset printing),or with a tool such as a doctor knife, a roll coater, a curtain coater,or a knife coater.

The display device displays an image by transmitting light from a lightsource or a display element. Therefore, the substrate and the thin filmssuch as the insulating film and the conductive film provided for thepixel portion where light is transmitted have light-transmittingproperties with respect to light in the visible-light wavelength range.

The first electrode layer and the second electrode layer (also referredto as the pixel electrode layer, the common electrode layer, the counterelectrode layer, or the like) for applying voltage to the displayelement may have light-transmitting properties or light-reflectingproperties, depending on the direction in which light is extracted, theposition where the electrode layer is provided, and the patternstructure of the electrode layer.

The first electrode layer 4030 and the second electrode layer 4031 canbe formed using a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (hereinafter also referredto as ITO), indium zinc oxide, or indium tin oxide to which siliconoxide is added.

The first electrode layer 4030 and the second electrode layer 4031 caneach be formed using one kind or plural kinds of materials selected frommetal such as tungsten (W), molybdenum (Mo), zirconium (Zr), hafnium(Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt(Co), nickel (Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper(Cu), or silver (Ag); an alloy thereof; and a metal nitride thereof.

A conductive composition containing a conductive high molecule (alsoreferred to as a conductive polymer) can be used for the first electrodelayer 4030 and the second electrode layer 4031. As the conductive highmolecule, a so-called π-electron conjugated conductive polymer can beused. For example, polyaniline or a derivative thereof, polypyrrole or aderivative thereof, polythiophene or a derivative thereof, a copolymerof two or more of aniline, pyrrole, and thiophene or a derivativethereof, or the like can be given.

Since the transistor is easily broken owing to static electricity or thelike, a protective circuit for protecting the driver circuit ispreferably provided. The protective circuit is preferably formed using anonlinear element.

By using any of the transistors described in Embodiments 1 to 4 asdescribed above, the display device can have a variety of functions.

This embodiment can be implemented by being combined with any of otherembodiments as appropriate.

Embodiment 6

A semiconductor device disclosed in this specification can be applied toa variety of electronic devices (including an amusement machine).Examples of the electronic device include a television set (alsoreferred to as a television or a television receiver), a monitor of acomputer or the like, a camera such as a digital camera or a digitalvideo camera, a digital photo frame, a mobile phone handset (alsoreferred to as a mobile phone or a mobile phone device), a portable gamemachine, a portable information terminal, an audio reproducing device,and a large-sized game machine such as a pachinko machine.

FIGS. 10A and 10B illustrate an example of an electronic book reader towhich a flexible semiconductor device formed according to any of theabove embodiments is applied. FIG. 10A illustrates an opened electronicbook reader, and FIG. 10B illustrates a closed electronic book reader.The flexible semiconductor device formed according to any of the aboveembodiments can be used for a first display panel 4311, a second displaypanel 4312, and a third display panel 4313.

A first housing 4305 has the first display panel 4311 including a firstdisplay portion 4301, a second housing 4306 has the second display panel4312 including an operation portion 4304 and a second display portion4307. The third display panel 4313 is a dual display panel and has athird display portion 4302 and a fourth display portion 4310. The thirddisplay panel 4313 is interposed between the first display panel 4311and the second display panel 4312. The first housing 4305, the firstdisplay panel 4311, the third display panel 4313, the second displaypanel 4312, and the second housing 4306 are connected to each other witha binding portion 4308 in which a driver circuit is formed. Theelectronic book reader of FIGS. 10A and 10B includes four displayscreens: the first display portion 4301, the second display portion4307, the third display portion 4302, and the fourth display portion4310.

The first housing 4305, the first display panel 4311, the third displaypanel 4313, the second display panel 4312, and the second housing 4306are flexible, and the flexibility of the electronic book reader is high.Further, when a plastic substrate is used for each of the first housing4305 and the second housing 4306, and a thin film is used for the thirddisplay panel 4313, a thin electronic book reader can be obtained.

The third display panel 4313 is a dual display panel including the thirddisplay portion 4302 and the fourth display portion 4310. For the thirddisplay panel 4313, either a display panel of dual emission type, ordisplay panels of one-side emission type which are attached to eachother may be used.

FIG. 11 illustrates an example in which a semiconductor device formed inaccordance with any of the above embodiments is used for an indoorlighting device 3001. Since the semiconductor device described in any ofthe above embodiments can be increased in area, the semiconductor devicecan be used for a lighting device having a large area. Further, thesemiconductor device described in any of the above embodiments can beused for a desk lamp 3000. Note that a lighting device includes, in itscategory, a wall light, a light for an inside of a car, an evacuationlight, and the like in addition to a ceiling light and a desk lamp.

In such a manner, the semiconductor device described in any ofEmbodiments 1 to 5 can be applied to various electronic devicesdescribed above, and highly reliable electronic devices can be provided.

This application is based on Japanese Patent Application serial no.2010-024385 filed with Japan Patent Office on Feb. 5, 2010, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising a transistor,the transistor comprising: a gate electrode; a first insulating layerover the gate electrode; an oxide semiconductor layer over the gateelectrode with the first insulating layer therebetween; a secondinsulating layer over the oxide semiconductor layer; and a conductivelayer over the oxide semiconductor layer with the second insulatinglayer therebetween, wherein the conductive layer is overlapped with thegate electrode and is electrically connected with the gate electrode,wherein each of the gate electrode and the conductive layer extendsbeyond both side edges of the oxide semiconductor layer in a channelwidth direction of the transistor, and wherein a value of an off-statecurrent per 1 μm of a channel width of the transistor is lower than 100zA/μm at 85° C.
 2. The semiconductor device according to claim 1,wherein the gate electrode and the conductive layer are in contact witheach other in both sides of the oxide semiconductor layer in the channelwidth direction of the transistor.
 3. The semiconductor device accordingto claim 1, wherein the oxide semiconductor layer comprises indium,zinc, oxygen, and a metal element.
 4. The semiconductor device accordingto claim 1, wherein the semiconductor device is a display device.
 5. Thesemiconductor device according to claim 1, wherein the transistor isincluded in a driver circuit.
 6. The semiconductor device according toclaim 1, wherein a carrier concentration of a channel formation regionof the transistor is lower than 1×10¹¹/cm³.
 7. The semiconductor deviceaccording to claim 1, further comprising: a source electrode over andelectrically connected with the oxide semiconductor layer; and a drainelectrode over and electrically connected with the oxide semiconductorlayer.
 8. A semiconductor device comprising a transistor, the transistorcomprising: a gate electrode; a first insulating layer over the gateelectrode; an oxide semiconductor layer over the gate electrode with thefirst insulating layer therebetween; a second insulating layer over theoxide semiconductor layer; and a conductive layer over the oxidesemiconductor layer with the second insulating layer therebetween,wherein the conductive layer is overlapped with the gate electrode andis electrically connected with the gate electrode, wherein each of thegate electrode and the conductive layer extends beyond both side edgesof the oxide semiconductor layer in a channel width direction of thetransistor so that the oxide semiconductor layer is surrounded by thegate electrode, the first insulating layer, the second insulating layerand the conductive layer, and wherein a value of an off-state currentper 1 μm of a channel width of the transistor is lower than 100 zA/μm at85° C.
 9. The semiconductor device according to claim 8, wherein thegate electrode and the conductive layer are in contact with each otherin both sides of the oxide semiconductor layer in the channel widthdirection of the transistor.
 10. The semiconductor device according toclaim 8, wherein the oxide semiconductor layer comprises indium, zinc,oxygen, and a metal element.
 11. The semiconductor device according toclaim 8, wherein the semiconductor device is a display device.
 12. Thesemiconductor device according to claim 8, wherein the transistor isincluded in a driver circuit.
 13. The semiconductor device according toclaim 8, wherein a carrier concentration of a channel formation regionof the transistor is lower than 1×10¹¹/cm³.
 14. The semiconductor deviceaccording to claim 8, further comprising: a source electrode over andelectrically connected with the oxide semiconductor layer; and a drainelectrode over and electrically connected with the oxide semiconductorlayer.
 15. A semiconductor device comprising a transistor, thetransistor comprising: a gate electrode; a first insulating layer overthe gate electrode; an oxide semiconductor layer over the gate electrodewith the first insulating layer therebetween; a second insulating layerover the oxide semiconductor layer; and a conductive layer over theoxide semiconductor layer with the second insulating layer therebetween,wherein the conductive layer is overlapped with the gate electrode andis electrically connected with the gate electrode, wherein the secondinsulating layer includes a first opening and a second opening where thefirst opening and the second opening are arranged in a channel widthdirection of the transistor in such a manner that the oxidesemiconductor layer is located between the first opening and the secondopening, wherein each of the gate electrode and the conductive layerextends beyond both side edges of the oxide semiconductor layer in thechannel width direction of the transistor so that portions of theconductive layer are formed in the first opening and the second opening,and wherein a value of an off-state current per 1 μm of a channel widthof the transistor is lower than 100 zA/μm at 85° C.
 16. Thesemiconductor device according to claim 15, wherein the gate electrodeand the conductive layer are in contact with each other in both sides ofthe oxide semiconductor layer in the channel width direction of thetransistor.
 17. The semiconductor device according to claim 15, whereinthe oxide semiconductor layer comprises indium, zinc, oxygen, and ametal element.
 18. The semiconductor device according to claim 15,wherein the semiconductor device is a display device.
 19. Thesemiconductor device according to claim 15, wherein the transistor isincluded in a driver circuit.
 20. The semiconductor device according toclaim 15, wherein a carrier concentration of a channel formation regionof the transistor is lower than 1×10¹¹/cm³.
 21. The semiconductor deviceaccording to claim 15, further comprising: a source electrode over andelectrically connected with the oxide semiconductor layer; and a drainelectrode over and electrically connected with the oxide semiconductorlayer.
 22. A semiconductor device comprising a transistor, thetransistor comprising: a gate electrode; a first insulating layer overthe gate electrode; an oxide semiconductor layer over the gate electrodewith the first insulating layer therebetween; a second insulating layerover the oxide semiconductor layer; and a conductive layer over theoxide semiconductor layer with the second insulating layer therebetween,wherein the conductive layer is overlapped with the gate electrode andis electrically connected with the gate electrode, wherein each of thegate electrode and the conductive layer extends beyond both side edgesof the oxide semiconductor layer in a channel width direction of thetransistor, and wherein a channel formation region of the transistorcomprises a crystal region which is c-axis-aligned perpendicularly to asurface of the oxide semiconductor layer.
 23. The semiconductor deviceaccording to claim 22, wherein the gate electrode and the conductivelayer are in contact with each other in both sides of the oxidesemiconductor layer in the channel width direction of the transistor.24. The semiconductor device according to claim 22, wherein the oxidesemiconductor layer comprises indium, zinc, oxygen, and a metal element.25. The semiconductor device according to claim 22, wherein thesemiconductor device is a display device.
 26. The semiconductor deviceaccording to claim 22, wherein the transistor is included in a drivercircuit.
 27. The semiconductor device according to claim 22, wherein acarrier concentration of a channel formation region of the transistor islower than 1×10¹¹/cm³.
 28. The semiconductor device according to claim22, further comprising: a source electrode over and electricallyconnected with the oxide semiconductor layer; and a drain electrode overand electrically connected with the oxide semiconductor layer.
 29. Asemiconductor device comprising a transistor, the transistor comprising:a gate electrode; a first insulating layer over the gate electrode; anoxide semiconductor layer over the gate electrode with the firstinsulating layer therebetween; a second insulating layer over the oxidesemiconductor layer; and a conductive layer over the oxide semiconductorlayer with the second insulating layer therebetween, wherein theconductive layer is overlapped with the gate electrode and iselectrically connected with the gate electrode, wherein each of the gateelectrode and the conductive layer extends beyond both side edges of theoxide semiconductor layer in a channel width direction of the transistorso that the oxide semiconductor layer is surrounded by the gateelectrode, the first insulating layer, the second insulating layer andthe conductive layer, and wherein a channel formation region of thetransistor comprises a crystal region which is c-axis-alignedperpendicularly to a surface of the oxide semiconductor layer.
 30. Thesemiconductor device according to claim 29, wherein the gate electrodeand the conductive layer are in contact with each other in both sides ofthe oxide semiconductor layer in the channel width direction of thetransistor.
 31. The semiconductor device according to claim 29, whereinthe oxide semiconductor layer comprises indium, zinc, oxygen, and ametal element.
 32. The semiconductor device according to claim 29,wherein the semiconductor device is a display device.
 33. Thesemiconductor device according to claim 29, wherein the transistor isincluded in a driver circuit.
 34. The semiconductor device according toclaim 29, wherein a carrier concentration of a channel formation regionof the transistor is lower than 1×10¹¹/cm³.
 35. The semiconductor deviceaccording to claim 29, further comprising: a source electrode over andelectrically connected with the oxide semiconductor layer; and a drainelectrode over and electrically connected with the oxide semiconductorlayer.
 36. A semiconductor device comprising a transistor, thetransistor comprising: a gate electrode; a first insulating layer overthe gate electrode; an oxide semiconductor layer over the gate electrodewith the first insulating layer therebetween; a second insulating layerover the oxide semiconductor layer; and a conductive layer over theoxide semiconductor layer with the second insulating layer therebetween,wherein the conductive layer is overlapped with the gate electrode andis electrically connected with the gate electrode, wherein the secondinsulating layer includes a first opening and a second opening where thefirst opening and the second opening are arranged in a channel widthdirection of the transistor in such a manner that the oxidesemiconductor layer is located between the first opening and the secondopening, wherein each of the gate electrode and the conductive layerextends beyond both side edges of the oxide semiconductor layer in thechannel width direction of the transistor so that portions of theconductive layer are formed in the first opening and the second opening,and wherein a channel formation region of the transistor comprises acrystal region which is c-axis-aligned perpendicularly to a surface ofthe oxide semiconductor layer.
 37. The semiconductor device according toclaim 36, wherein the gate electrode and the conductive layer are incontact with each other in both sides of the oxide semiconductor layerin the channel width direction of the transistor.
 38. The semiconductordevice according to claim 36, wherein the oxide semiconductor layercomprises indium, zinc, oxygen, and a metal element.
 39. Thesemiconductor device according to claim 36, wherein the semiconductordevice is a display device.
 40. The semiconductor device according toclaim 36, wherein the transistor is included in a driver circuit. 41.The semiconductor device according to claim 36, wherein a carrierconcentration of a channel formation region of the transistor is lowerthan 1×10¹¹/cm³.
 42. The semiconductor device according to claim 36,further comprising: a source electrode over and electrically connectedwith the oxide semiconductor layer; and a drain electrode over andelectrically connected with the oxide semiconductor layer.